The present invention relates to a braking system for a vehicle. In particular, the present invention relates to the provision of an automatic traction control (ATC) function and a roll stability (RS) function with a vehicle braking system.
A typical vehicle braking system for a straight truck, bus, tractor, or trailer, includes a source of pressurized air along with valves for selectively directing the air to brake chambers at the wheels of the vehicle. Many such vehicle braking systems provide an antilock braking system (ABS) function, by which an electronic control unit (ECU) selectively releases and applies braking at individual wheels to prevent wheel lockup.
Some vehicle braking systems also provide an automatic traction control (ATC) function. In one aspect of ATC, an electronic control unit (ECU) selectively applies braking at individual wheels to match wheel speeds side to side to help control wheel spin that occurs in response to driver demand via the accelerator pedal. This control is typically effected by controlling a wheel end modulator associated with the wheel. The modulator provides an air flow path to the wheel that can be rapidly opened or closed by a solenoid under the control of the ECU.
In order to provide the ATC function, high pressure air must be made available at the modulators in the absence of driver demand. This is typically done by having a constant supply of high pressure air from the reservoir to an ATC solenoid that is associated with the modulators on the driven axle. In an ATC event, the ATC solenoid is energized under the control of the ECU to direct the high pressure air from the reservoir to the modulators. The modulators are then controlled by the ECU selectively to apply and release braking force to the wheels, to control any wheel spin.
As one example, FIG. 1 shows schematically some portions of a prior art vehicle braking system that is operative to provide ATC on a straight truck, or bus. The system is also ABS effective. The system includes a foot brake valve (FBV) that normally provides driver demand proportioned air pressure as a control air flow to a relay valve (designated ATC) associated with the driven wheels of the vehicle. The relay valve includes an ATC solenoid that is controlled by an ECU. The ATC solenoid is normally de-energized, in which case driver demand proportioned air is used as the control pressure on the relay valve, to control the flow of air from the reservoir to the rear axle modulators. When the ECU determines that the traction control function is desired, the ECU energizes the ATC solenoid on the relay valve, which opens the relay valve immediately, in spite of the lack of driver demand controlled pressure. Reservoir air pressure is provided to the rear modulators. The rear modulators, under the control of the ECU, use that air to modulate brake pressure in the brake actuators, thus controlling the spinning of the rear wheels.
Vehicle air brake systems typically include a primary circuit, which is often used for driven wheels, and a secondary circuit, which is often used for non-driven wheels. The vehicle has a dual brake valve, or foot brake valve (FBV), which is a valve in both the primary circuit and the secondary circuit that is controlled by the foot pedal (brake pedal) of the vehicle in response to driver demand for braking. The dual brake valve is supplied with high pressure air from one or more reservoirs. When the valve is actuated by driver applied force on the brake pedal, this high pressure air is directed into the primary and secondary braking circuits of the vehicle.
Some vehicles with dual brake valves also have a brake valve actuator (BVA), which is an actuatable device interposed between the valve and the brake pedal that applies enough force to the brake valve so that the brake valve's output (delivery pressure to the primary and secondary circuits) is at least 85 psi. The BVA is operated (piloted by, or receives a pneumatic control signal from) either (a) a pneumatic on/off control valve actuated by the driver's hand like a switch, or (b) an on/off solenoid valve controlled by an on/off electrical switch operated by the driver's hand. The BVA is used to conduct a pre-trip inspection, in which the brakes are set and held in an applied condition so that the driver can check the braking system of the vehicle without having to be in the cab pressing on the brake pedal. U.S. Pat. No. 6,659,244 shows the use of a brake valve actuator in a vehicle air braking system.